Polymer particles encompass an important technology which has found uses in a very wide variety of areas including paints, protective coatings, adhesives, matting agents, absorbent materials, additives for building materials, drug delivery systems, sizing agents, ink additives, bioanalytical systems, cosmetics, and chromatography materials. Polymer particles are usually synthesized in heterogeneous systems consisting of an organic phase and an aqueous phase resulting in an aqueous dispersion of the polymer particles. Colloidal stability is imparted to the particles by a dispersing compound that is usually a surfactant or an oligomeric or polymeric amphiphile. Many of the properties of the polymer particle and its aqueous dispersion are dictated or influenced by the choice of dispersing compound used. These properties include stability of the dispersion to aging, settling, shear, extremes of temperature, and the presence of other materials such as electrolytes, humectants, and flocculants. The choice of dispersing compound will also influence the size and morphology of the particles as well as the rheology of the dispersion, its propensity toward foaming, and the amount of coagulum formed during the polymerization process.
Nonionic surfactants represent an important class of dispersing compounds. These compounds are desirable in that they afford particles a much higher tolerance toward electrolytes than do ionic dispersants. It is known in the art, however, that the overall colloidal stability of the particles toward other conditions (aging, heating, high shear, etc.) is usually much lower than for particles stabilized with ionic surfactants. Thus, it is commonly practiced in the art of heterogeneous polymerizations, (notably emulsion polymerizations) that nonionic surfactants are used in tandem with an ionic surfactant (usually anionic) in order to gain sufficient colloidal stability. Thus there is a need in the art of heterogeneous polymerization for a nonionic dispersant which yields particle dispersions with colloidal stability which is comparable to those obtained with ionic surfactants.
Miniemulsion (also known as “microsuspension”) polymerizations are a specific type of heterogeneous polymerization in which a chain polymerization occurs within a pre-formed oil-in-water dispersion in which the monomers are contained within the oil phase. It is known that for this type of polymerization (see Sudol, E. D. and El-Aasser, M. in Emulsion Polymerization and Emulsion Polymers; Lovell, P. A. and El-Aaser, M. Eds.; John Wiley and Sons Ltd.: New York, 1997; p. 699-721) a “cosurfactant” is needed in order to prevent Ostwald ripening of the particles. This cosurfactant is a hydrophobic compound, such as hexadecane or hexadecanol. This component, though necessary, may be unwanted for certain applications in which its presence might be deleterious. Thus there also exists a need for a surfactant useful for miniemulsion polymerization, which alleviates the need for a cosurfactant.
Hydrophobe-capped polyacrylamide oligomers have been reported as milling agents for pigment particles in inkjet inks, as stabilizers for photographic coupler dispersions, and as stabilizers for organosilver particles. (U.S. Pat. Nos. 6,127,453, U.S. Pat. No. 2001/0031436 A1 and U.S. Ser. No. 09/776,107). These applications, however, each involve the stabilization of species that are not polymer particles (i.e. silver behenate particles, pigmented colorants, and photographic coupler solvents). It is known in the art that different classes of chemical species will have fundamentally different requirements for dispersants and the use of these dispersants in these applications would not suggest their usefulness in stabilizing polymer particles.
In Comun. Jorn. Com. Esp. Deterg. (1995), 26, 323-35, the authors report the use of polyacrylamide as a dispersant in an inverse emulsion polymerization. This compound, however, is a different chemical composition than the hydrophobically capped polyacrylamide oligomers reported herein and would be expected to have fundamentally different activity as a stabilizer of polymer particles.
The object of this invention is to provide dispersant-stabilized polymer particles which show improved excellent colloidal stability when dispersed in aqueous media.